1. Field of the Invention
This invention relates to an imaging apparatus for endoscopes in which an objective optical system is provided with optical filters which reduce a transmittance in a particular wavelength region of illumination light irradiating an object to 0.1% or less, and in particular, to an imaging apparatus for endoscopes in which illumination light irradiating an object is excitation light inducing fluorescent light and an objective optical system is provided with fluorescence observing optical filters which reduce a transmittance in the wavelength region of the excitation light to 0.1% or less.
2. Description of Related Art
Recent developments have involved the use of techniques that auto-fluorescence from a living body or fluorescence of medicine administered to the living body is detected as a two-dimensional image by an endoscope, and states of the degeneration of a biochemical tissue and a disease, such as cancer, (for example, the kind of disease and a penetration area) are diagnosed from this fluorescent image.
FIG. 1 shows one conventional example of an endoscope system for observing fluorescence. The endoscope system of this example includes an endoscope 21, a signal processing means 22, a light source means 23, and a monitor 24. Excitation light from the light source means 23 is conducted to a distal end portion 25 of the endoscope 21 to irradiate a living body, not shown. Fluorescent light emanating from the living body is imaged by an imaging apparatus for endoscopes placed at the distal end portion 25 and its electric signal is converted by the signal processing means 22 so that an image can be observed through the monitor 24.
Conventional examples of endoscopes for observing fluorescence are set forth, for instance, in Japanese Patent Kokai Nos. Hei 9-70384, 2002-153414, and 2002-10969.
As disclosed in these prior art articles, fluorescent light, in contrast with excitation light, is very faint, and thus, to observe the fluorescent light, it is necessary that an optical filter which cuts off the excitation light and transmits the fluorescent light is placed in an objective optical system of the imaging apparatus for endoscopes.
Japanese Patent Kokai No. Hei 11-223726 discloses an interference film filter in which the transmittance of excitation light is 0.1% or less and suggests that since excitation light incident on the filter is sufficiently cut off with respect to fluorescent light, a fluorescent image of good contrast is obtained.
The conventional examples, however, fail to describe the fact that excitation light passes through a mechanical clearance between the optical filter cutting off the excitation light and a holding frame retaining the optical filter and the observation is seriously affected, and provision for this.
An example of a common objective optical system in the imaging apparatus for endoscopes of this type is illustrated in FIG. 2. In the objective optical system of an imaging apparatus 14 for endoscopes in FIG. 2, lenses L1, L2, L3, and L4; optical filters F1, F2, F3, F4, and F5; stops 7, 8, 9, 10, and 11; and spacer rings 4, 5, and 6 are incased in a lens frame 1 so that the outside surface of the lens L1 and the inside surface of the lens frame 1 are fixed by cementation and the outside surface of the filter F5 and the inside surface of the lens frame 1 are also fixed by cementation. Thus, a clearance is necessarily provided between the outside surfaces of other lenses, other optical filters, the stops, and the spacer rings and the inside surface of the lens frame 1. Also, in FIG. 2, reference numeral 2 denotes a solid-state image sensor frame, symbol F6 denotes a filter or cover glass, and numeral 3 denotes a solid-state image sensor.
FIG. 3 shows one example of a course where excitation light passes through the clearance between the optical filers and the lens frame in FIG. 2. In FIG. 3, the lens frame and the solid-state image sensor frame are omitted. Rays of light refracted in the proximity of the periphery of the image-side curved surface of the lens L2 are nearly parallel with the optical axis of the objective optical system and attain heights corresponding to the outside diameters of the optical filters. The rays travel in straight lines through the clearance between the outside surfaces of the optical filters F2–F4, the stops 8–11, and the spacer rings 4 and 5 and the inside surface of the lens frame retaining them and are refracted by the lenses L3 and L4, reaching the solid-state image sensor 3. Since this propagation light travels in straight lines without undergoing reflection and absorption by the lens frame and the spacer rings, the amount of light is not diminished. However, when one of the optical filters F2–F4 is placed as an excitation light cutoff filter, the rays refracted in the proximity of the periphery of the image-side curved surface of the lens L2 miss the optical filters and pass through the clearance, and thus excitation light to be originally cut off reaches the solid-state image sensor. Consequently, the contrast of the fluorescent image is deteriorated and the observation is obstructed.
For example, when the outside diameters of the optical filters F2–F4, the stops 8–11, and the spacer rings 4 and 5 are assumed to be 2 mm, the inside diameter of the lens frame retaining the optical filters is set to approximately 2.05 mm, allowing for an insertion of them into the lens frame 1. When the inside diameters of the aperture stops are 0.96 mm, the area of the clearance is 20% of the area of the inside diameter of each of the aperture stops. When the transmittance of excitation light of the optical filters F2–F4 is assumed to be 0.1%, there is the possibility that the intensity of excitation light passing through the clearance becomes about 200 times that of excitation light transmitted through the optical filters and the S/N ratio is reduced to nearly 1/200.
FIG. 4 shows another example of a course of light where an image of an object is formed in an objective optical system unlike that of FIG. 2. FIG. 5 shows another example of a course where excitation light passes through the clearance between the optical filers and the lens frame in FIG. 4. Also, the lens frame and the solid-state image sensor frame are omitted from the figures. In FIG. 4, lenses L5–L7 and optical filters F7–F9 of the objective optical system are incased in the lens frame, not shown.
In FIG. 4, when one of the optical filters is the excitation light cutoff filter, rays refracted in the proximity of the periphery of the image-side curved surface of the lens L5 become nearly parallel with the optical axis and travel in straight lines through the clearance between the outside surfaces of the optical filters F7–F9 and the inside surface of the lens frame, not shown. As such, the above problem is not limited to a particular lens arrangement. That is, this problem arises when the outside diameter of an object-side lens close to the excitation light cutoff filter is larger than that of the excitation light cutoff filter.
As another factor of reducing the contract of fluorescent image, there is the phenomenon that part of fluorescent light transmitted through the excitation light cutoff filter and imaged on the imaging surface of the solid-state image sensor repeats reflection between the imaging surface and the optical filters and is reimaged on the imaging surface, following another course.
In order to obtain the fluorescent image of good contrast, as mentioned above, it is necessary to solve new technical problems in addition to already known techniques.